Regular Expression :: Check A Text Field Value To Accept Between 0-20

i'm fairly new to regular expression and want to know if it's possible to check a text field value. I need to only accept value between 0 and 20. I've got also a problem with the 0 value ... guess the script think it's false :

if(!Number(document.data.f_cat_1.value) || document.data.f_cat_1.value > 20){
alert('Your must enter a numeric value between 0 & 20 pts!');
document.data.f_cat_1.focus();
return false;
}